cmake/cpack_rpm.cmake:
  * mark all cnf files with %config(noreplace)
  * add the forgotten postun script
sql/sys_vars.cc:
  0 for a string variable means "no default. But datadir has the default value.
support-files/rpm/server-postin.sh:
  * use mysqld --help to determine the correct datadir in the presence of my.cnf files
    (better than my_print_defaults, because it considers the correct group set).
  * Only create users, and chown/chmod if it's a fresh install, not an upgrade.
  * only run mysql_install_db if datadir does not exist